1. Energetics of reproduction in birds is reviewed with the question in mind how the parent adjusts its effort in relation to prevailing environmental conditions in order to maximize the output of young in its lifetime. Emphasis is on proximate controls, rather than ultimate factors measurable in terms of adult survival and recruitment of young. 2. The decision to breed or not to breed is clearly related to body condition of the female, presumably because of the implications this has for survival (see Fig. I). 3. Laying date and clutch size are likewise under the influence of female condition and can hence be modified by experiments involving supplementary feeding (Fig. 2, 3). Natural variation in these features may often be related to territory quality (Fig. 4). 4. How the bird decides whether or not to commence a second brood is not clear, but in the Great Tit the habitat-related difference in incidence of second broods is functionally understandable when survival probabilities of birds at different times are considered (Fig. 5). 5. A distinction is made between a 'capital' and 'income' model for translating rates of change of female body condition into appropriate decisions on laying date and clutch size (Fig. 6) and experiments are suggested that discriminate between the two. 6. Lack's view that brood size is in an evolutionary sense adjusted in order to balance food requirement and foraging capacity of the parents is accepted, and growth rates in nidicolous birds are analysed to ascertain if a finer adjustment exists superimposed on the integer steps of brood adjustment. Critical for this analysis are groups of birds where broods of one are common, since only in these circumstances is growth adjustment the only strategy open to the parents. In common with other animals, growth rate is related to mature body size (Fig. 8) but within a category of adult weight clear examples can be found for retardation of growth rate in pelecaniform and charadriiform species with singleton broods. 7. Since daily energy requirement is related to nestling size (Fig. 9) and growth rate (Fig. 10), retardation of growth is explicable as a strategy only in terms of reducing the daily commitment of the parents, not reducing the total cost of producing a nestling. 8. An additional economy in growth is to reduce the contribution of fat to the nestling body (Fig. II). 9. Implied in Lack's view of brood size is a limitation of parental foraging capacity, and the last section of the paper is devoted to exploration of the proximate factors delimiting what Royama terms the optimal working capacity of parents feeding young. Observations of parent starlings confronted with manipulated brood size suggest a limit on the time that can be devoted to energetically extravagant flight activity, rather than a shortage of absolute time (Fig. 12). Beyond the limit to which stressed parents can be made to fly, body weight declines (Fig. 13). 10. Preliminary data on energy metabolized daily by parents confronted with large broods conforms to the simplified view that parental effort on a sustained basis equates to energy mobilization equivalent to 4 B.M.R. units (Fig. 14) and it is suggested that this level of energy expenditure represents a proximal decision substrate for determining the optimal working capacity of the parent. II. The paper ends with a plea for more research on the proximate controls of avian reproduction, and calls attention to the central importance of the protein bank to parental body condition.